Wearable + Phone GPS?

Fitbit adds route tracking via phone GPS

I’ve just received an email from Fitbit. As we discussed for Compass H2O, they’re now offering Route Tracking, not by modifying the Fitbit device at all, but by using your phone GPS. What I don’t quite understand in Fitbit’s case is: if I’m going to track with my phone, why do I still need their device at all?